Fatal error: Class 'Debug' not found

I have two ss websites installed on the same server, and both are working just fine. One of them is currently under development. I made some changes yesterday and checked everything was right. However, This morning the frontpage shows the following error for the first time. I checked the other website to find the same error on the frontpage! :

Fatal error: Class 'Debug' not found in /home/bohoich/public_html/sapphire/core/Core.php on line 225

the core file is still there, and I didn't create any class. I run dev/build and it shows a list of ManifestBuilder.php errors. The strange thing is it affects another website on the server with ss 3.4 version and shows the same error. That made me think that maybe something is wrong with server, so I went to the hosting support told me they are fixing something with the server.. now I need to wait to see how they are going to fix this..

I have a reseller account with 5 websites using only 2% of resources. Three websites are silverstripe v2.4, and they suddenly show the above same error. I thought it's a problem with the server but so far the support couldn't do anything about it. They added a php.ini file in public_html folder and asked me to enable the module I want inside that file, but I wasn't sure what to do exactly..!

the error shows on a white screen not the usual ss blue screen, so maybe something was disabled or misconfigured for php execution which affects only silverstripe websites.